At present all over the world in production of general-purpose steel use is made of ferroalloys as alloying materials.
The use of ferroalloys requires large energy expenditure since an energy carrier is consumed in great amounts. Besides, the production of ferroalloys is accompanied with the formation of harmful for the environment gases in concentration exceeding maximum permissible levels in the atmosphere according to the international standards.
The production of general-purpose steel with the use of ferroalloys proceeds with high losses of an alloying element and with a low degree of extraction thereof into steel. This is due to the fact that the process of preparing ferroalloys and the process of steel alloying are high-temperature and are accompanied with high losses of the alloying element together with gaseous products of the reaction.
In addition, in the production of ferroalloys there exists a high slag multiplicity also resulting in high losses of the alloying element with the slag.
The degree of extraction of the alloying element from ferroalloys into steel is low since approximately one fifth of the ferroalloy is consumed for deoxidizing of metal.
Known in the art is a method of steel refining by the introduction of oxide materials containing metals, which are alloying for the steel being smelted, into a steel making unit (JP, A, 59-215412).
The known method includes the loading of cast iron into a smelting unit, the subsequent supply of oxide materials containing alloying metals, for instance, manganese, chromium, silicon, molybdenum, vanadium, niobium, and cobalt simultaneously with a carbon-containing material into the same unit, and delivery from below of a gas-oxygen mixture containing 20-70% of oxygen at a flow rate of 0.1 mm.sup.3 /t.multidot.min.
This is accompanied with a decrease in the amount of carbon in the carbon-containing material and with liberation of heat required for heating and melting all the materials introduced into the steelmaking unit.
Besides, a part of the carbon is consumed for the reduction of the alloying elements from the slag which proceeds with high heat losses. As a result, mild steel is obtained.
Since oxide materials are used without a thermal treatment, they contain hydrate moisture and readily dissociating complex compounds of various oxides. When such oxides find their way to the surface or into the bulk of liquid metal, gaseous dissociation products are formed the liberation of which leads to slag foaming and to discharge of the metal and slag from the steel-making unit.
As a reducer in the known method use is made of carbon-containing materials introduced into the steel-making unit simultaneously with oxide materials.
It is known that the reaction of carbon with oxygen is endothermal; therefore, to compensate for heat losses in the course of the reduction, an additional heating of liquid metal is required (prior to introducing oxide and carbon-containing materials) up to a temperature higher than that in the method of steel alloying with ferroalloys.
Besides, a simultaneous supply of oxide and carbon-containing materials and a subsequent blowing of liquid metal with an oxygen-containing gaseous mixture results in combustion of carbon in the flow of the gaseous mixture, hence, in an inefficient consumption thereof as a reducer.
The use of the known method does not ensure any certain degree of extraction of the alloying element, for instance, manganese, from the oxide material into steel since it is impossible to predict what amount of the carbon-containing material is consumed for the reduction and what amount is burnt.
Largely, the rational use of manganese in the known method depends on the final carbon content in the steel obtained. If carbon content is less than 0.2%, the manganese extraction degree is no more than 60%.
The known method can be applied only on a limited scale because of the need to have a special steelmaking unit with a bottom blowing, to use as a metal charge an iron cast refined from sulphur, phosphorus, and silicon, the temperature of the iron cast after the completion of the refining process being no more than 1250.degree. C. against 1350.degree.-1400.degree. C. of conversion pig iron, and the use of a high-grade manganese ore with a manganese content of up to 50%.
All of the above features make steel more expensive because of the necessity to use a special steelmaking unit not always justified in production of general-purpose steel, an enhanced consumption of heat carriers to compensate for heat losses in iron cast refining, and expensive high-grade manganese ore.
Known in the art is a method of steel alloying with manganese (SU, A, 104464) involving the smelting of a carbon semiproduct, tapping said semiproduct into a ladle, delivery into the ladle onto the surface of the liquid semiproduct of a thermally pretreated oxide material containing an alloying element. A low-phosphorus manganese slag (LPS) of ferroalloy production, is used as an oxide material, aluminium as a reducer, and lime in an amount ensuring the basicity of the slag equal to 2.0-3.5. Then the surface is blown-out with oxygen for 3-30 s and then blown-through with argon.
This method, however, does not provide steel of a high grade since the oxide material containing manganese as an alloying material (low-phosphorus manganese slag of ferroalloy production), reducer, and lime are delivered into a ladle after tapping the carbon semiproduct therein, after which oxygen is fed.
The above-described delivery of the materials into the ladle complicates the control of the steel alloying process and does not ensure the conditions for removing nonmetallic inclusions from the bulk of the liquid metal formed as a result of deoxidizing the carbon semiproduct with the reducer. Oxygen supply makes the use of the reducer inefficient and enhances the losses of the reducer on ignition, thereby deteriorating the quality of the final steel due to an increase of the amount of non-metallic inclusions in the steel.
A simultaneous delivery of the materials into the ladle after tapping the carbon semiproduct therein makes the processes of manganese reduction from the slag melt, deoxidation, and alluminium alloying of the metal also simultaneous. As a result an aluminium-alloyed steel is obtained and there is no possibility to obtain rimming steel.
Besides, a simultaneous delivery of the materials into the ladle after tapping the carbon semiproduct therein decreases the degree of extraction of an alloying element (manganese) fromn the slag melt and makes the use of the reducer inefficient as a result of which the content of manganese in the final steel is lowered.
All the above deteriorates quality of the final steel.
This method does not ensure the preparation of a high-grade rimming steel since:
1) the supply of the reducer into the ladle after tapping the carbon semiproduct is accompanied with deoxidation of the metal which deteriorates the rimming of steel after pouring and, hence, enhances the surface defects of the rolled products prepared from such steel;
2) the argon blowing-through of the metal in the ladle decreases the oxygen content in the steel which also deteriorates the steel quality because of a poor rimming of the metal after pouring;
3) the use of a low-phosphorus manganese slag (LPG) of ferroalloy production as an alloying additive decreases the oxygen content in the steel prior to pouring since LPS has low reducibility. This is due to the fact that LPS contains hardly reducible manganese silicates whose destruction and subsequent reduction of manganese from oxides require a high temperature (of about 1600.degree. C.) and, hence, an additional consumption of energy carrier. The melting of LPS requires an additional time period and in this case the reducer introduced into the ladle is consumed mainly for deoxidation of the carbon semiproduct and to a lesser extent is used for its designated purpose, namely, as a reducer of the alloying element (manganese) from the slag melt.
This results in a decrease in the oxygen content in the steel prior to pouring, and in an increase in the amount of nonmetallic inclusions. After pouring, the rimming of steel deteriorates which worsens the quality of the final steel and enhances rolling defects.
Besides, when using the known method, it is very difficult to prepare steel with narrow concentration limits of manganese and aluminium since the delivery of the oxide material and reducer into the ladle is not regulated with respect to the amount and sequence thereof. According to this method, the reducer and manganese-containing oxide material are delivered just after tapping the carbon semiproduct into the ladel because of which manganese reduction, deoxidation, and aluminium alloying of the carbon semiproduct proceed simultaneously. Since all the materials are fed into a metal bath with the unknown and unregulated oxidation level of the carbon semiproduct, the method has the same disadvantages as those observed in the course of usual deoxidation of metal with aluminium, i.e. the assimilation degree of the part of aluminium consumed for metal deoxidation and alloying varies within wide limits. In this case the steel obtained may have both a very low and a very high aluminium content, but it is impossible to prepare steel with a preset aluminium content.
In the known method the consumption of manganese-containing materials is also unregulated which, along with uncontrolled assimilation of aluminium, complicates the preparation of steel with a narrow preset range of manganese content and decreases the grade of the final steel.
In the production of steel by the known method the capacity of the steelmaking unit decreases as compared with the process of steel alloying with manganese ferroalloys since the delivery of all the materials into the ladle just after tapping the carbon semiproduct makes the processes of alloying and melting more prolonged. This leads to a decrease in the capacity and makes steel more expensive.
In addition, the manganese-containing oxide material used in the known method is expensive since it is prepared in electric-arc furnaces, i.e. the production requires a great amount of energy and heavy capital outlays for the equipment. The use of this material, naturally, makes steel more expensive. Low extraction (about 80%) of the alloying element (manganese) and losses of aluminium (more than 20%) because of an inefficient use thereof increase the cost of steel even more.
Besides, the known method provides the preparation of only manganese-alloyed steel whereas steel alloyed with other elements cannot be obtained.
A simultaneous introduction of oxide materials and reducer is accompanied with high losses of the reducer on ignition. This is due to the fact that the reducer is a light material and, reacting with the atmospheric oxygen, accumulates on the surface of the bath of the slag melt since the reducer has a greater affinity to oxygen than the alloying element entering into the composition of the oxide material.
The reducing materials have a lower melting point than the oxide materials containing an alloying element; therefore, being simultaneously introduced onto the liquid metal surface, the reducers melt in the first turn. The density of the reducers is more than two times lower than that of liquid steel; therefore, they melt on the surface of the liquid metal. In the presence of iron oxides on the surface of liquid metal the melting process is usually accompanied with the formation of gaseous products of incomplete oxidation of the reducers. This results in an inefficient consumption of the reducers and in a decrease of the degree of extracting the alloying element from the oxide material into steel, thereby making the production of steel more expensive.
The use of the known method does not allow one to obtain rimming steel since the blowing with argon decreases the oxygen content in steel and deteriorates the quality because of poor rimming in the course of pouring.
In the production of aluminium-alloyed steel by the known method the consumption of a reducer must be considerably enhanced. A rise in the reducer consumption, however, contaminates the metal with nonmetallic inclusions, namely, the products of reduction of oxide materials containing an alloying element. This, in its turn, worsens the quality of the steel produced.